1. Field of the Invention
The invention relates to a projection objective of a microlithographic projection exposure apparatus. Such apparatuses are used for the production of large-scale integrated electrical circuits and other microstructured components.
2. Description of Related Art
One of the essential aims in the development of projection exposure apparatuses is to be able to lithographically define structures with smaller and smaller dimensions on a photosensitive layer. Small structures lead to high integration densities, and this generally has a favorable effect on the performance of the microstructured components produced with the aid of such systems.
The generation of particularly small structure sizes re-quires a sufficiently low resolution of the projection objectives. Since the resolution of the projection objectives is proportional to the wavelength of the projection light, successive product generations of such projection exposure apparatuses use projection light with shorter and shorter wavelengths. The shortest wavelengths used at present lie in the ultraviolet spectral range and are usually 193 nm, in a few cases 157 nm.
However, the increasingly short wavelengths and high light powers of the lasers used as projection light sources lead to greater photo-induced ageing phenomena in the optical elements of such projection exposure apparatuses, which degrade the imaging quality. In projection objectives degradations of the imaging quality are tolerable only within extremely narrow limits. The extent to which the ageing progresses also depends on the particular conditions under which the components are produced. Relevant factors include the type of reticle used and the illumination setting selected, i.e. the angular distribution of the projection light illuminating the reticle.
The physical causes for the occurrence of ageing phenomena are manifold and depend primarily on the materials of which the optical elements and/or their coating consist. With high radiation densities, for example, quartz glass commonly used at wavelengths of 193 nm becomes gradually converted—albeit to a small degree—into a crystalline phase which has a higher refractive index than amorphous quartz glass. Extensive transcrystallisation and recrystallisation processes are also observed in the sensitive coatings which are generally applied to refractive optical elements in order to decrease the reflectivity, and to reflective optical elements in order to increase the reflectivity. A common aspect of such material modifications is that they cannot be eliminated per se and therefore cause irreversible damage to the optical elements.
In order to prevent the occurrence of such irreversible damage, it has been proposed to extend the pulse lengths of the lasers used as a light source. This can at least delay those ageing phenomena which are due to nonlinear optical effects, such as may occur at the start and end of each light pulse.
Attempts are also being made to find alternative materials which exhibit less pronounced ageing phenomena at the wavelengths contemplated. Examples of these are calcium fluoride (CaF2), which has the advantage of still being sufficiently transparent not only at a wavelength of 193 nm but also at a wavelength of 157 nm. However, it is difficult to produce this cubic crystalline material with the requisite purity, so that the few available crystals are very expensive. Calcium fluoride furthermore has some unfavorable properties, for example intrinsic birefringence, so that its use as a lens material necessitates special measures which are very elaborate especially at the wavelength of 157 nm.
Once the imaging quality of a projection objective has deteriorated intolerably because of damage attributable to ageing phenomena, it is in principle possible to replace the affected optical elements. However, replacing a plurality of optical elements entails high costs and production delays. At least for cost reasons, therefore, it may be favorable if imaging errors due to ageing phenomena are subsequently corrected by suitable measures. Relevant examples include the realignment of individual optical elements with the aid of manipulators which are known per se. Unfortunately, the category of imaging errors which can be corrected in this way is very limited.
One way to correct small but otherwise almost arbitrary perturbations of the wavefront profile is to apply local surface deformations on selected surfaces of individual optical elements. The surface deformations are designed such that a perturbed wavefront is restored to the intended profile by local phase changes when it passes through the surface deformations.
When worn-out optical elements are replaced or re-processed it is necessary to take the projection exposure apparatus out of operation for some time. The projection exposure apparatus cannot be used for the production of microstructured components during a period which may last days or even weeks owing to the complexity of the projection objectives. This leads to great financial losses.